This goal of this study is to determine if there is an underlying hormonal basis for the variations in orthostatic tolerance in a subset of women who are orthostatically intolerant. Based on the hypothesis, supported by anecdotal reports and preliminary data that some female subjects are orthostatically much more intolerant during certain phases of their menstrual cycle, we plan to study the influence of the menstrual cycle in a subset of 10 orthostatically intolerant women, 20 orthostatically tolerant women, and 10 female patients with POTS. The female patients will undergo autonomic testing at 3 phases of their menstrual cycle for 3 months resulting in 9 evaluations. The phases will be menstrual (second or third day of the cycle), follicular pre-ovulatory (1 day before the expected day of ovulation), pre-menstrual luteal (7 days prior to day of ovulation), pre-menstrual luteal (7 day sprior to menstruation). The oral temperature will be measured each morning at the same time before arising. The last low point in the temperature curve before the rise towards the hyperthermic plateau started is considered the day of ovulation. Serum levels of 17 beta-estradiol, progesterone, serum luteinizing hormone, and follicule stimulating hormone will be measured on the morning of these 9 times. The hormone levels provide distinctive profiles of these times. We will study 17 b-estradiol, progesterone, serum luteinizing hormone and follicular stimulating hormone using RIA. Women to be evaluated must have a regular and predictable menstrual cycle and must not be taking oral contraceptive agents. The evaluation on each occasion consists of the response to tilt of 20 minutes during (cardiovascular indices), the HR response to 8 cycles of deep breathing at a rate of 6 breaths/minute and the HR and BP response to Valsalva maneuvers performed while supine. The study will be done over 3 months for each subject. On each visit, the subject, clothed in a light hospital gown, is weighed. A QSART (quantitative sudomotor axon reflex) will be performed after in the 3 visits of the fist month. A semiquantitative symptoms scale will be obtained during tilt. Patients will be graded on dizziness, tremulousness, and anxiety. The study is ongoing. Studies on control subjects are on schedule. Studies on patients with orthostatic intolerance have been more difficult, since these patients have to return for 9 visits on short notice. However, recruitment has improved over the last few months.